1. Field of the Invention
The present invention relates to an oscillator having a feedback circuit and amplifiers using a piezoelectric vibrator, and to a production method therefor.
2. Description of the Related Art
In oscillators comprising a feedback circuit and amplifiers using a piezoelectric vibrator, piezoelectric ceramic compositions having lead titanate zirconate (Pb(TixZr1-x)O3) or lead titanate (PbTiO3) as the main ingredients thereof, are in widespread use as piezoelectric vibrators.
However, since piezoelectric ceramic compositions having lead titanate zirconate or lead titanate as the main ingredients thereof contain a large amount of lead therein, a problem has arisen in that the uniformity of products decreases because of the evaporation of lead oxides in the production process.
To prevent this problem from occurring, it is desirable to use a piezoelectric ceramic composition that does not contain therein any lead at all, or that contains therein only a little lead. Also, from the viewpoint of the environmental pollution problem, the less the content of lead, the further environmental friendliness is improved.
Accordingly, as a substitute for a piezoelectric ceramic composition having lead titanate or the like as the main ingredient thereof, it has been thought of using a bismuth layered compound such as SrBi2Nb2O9 or CaBi4Ti4O15. Since the piezoelectric ceramic composition having a bismuth layered compound as the main ingredient thereof contains therein no lead oxides, there is no risk that it raises the above-described problem.
In general, the oscillation frequency of an oscillator comprising a feedback circuit and amplifiers using a piezoelectric vibrator changes with the change in temperature. One of the major factors of this frequency change, the change in the resonant frequency of the piezoelectric body with temperature.
It is generally desirable, therefore, to use a piezoelectric body having a minimum of temperature-change of resonant frequency. However, in a piezoelectric ceramic composition having, as the main ingredient thereof, a bismuth layered compound such as SrBi2Nb2O9 or CaBi4Ti4O15, the temperature change rate fr-TC of the resonant frequency at xe2x88x9220 to 80xc2x0 C., which is the working temperature range for common oscillators, is high. Accordingly, such a piezoelectric ceramic composition is difficult to use as a piezoelectric vibrator for an oscillator requiring a high oscillation frequency accuracy.
frxe2x88x92TC=(fr(max)xe2x88x92fr(min))/(fr(20)xc2x7100) 
Here, fr(max) is the maximum resonant frequency in the temperature range from xe2x88x9220 to 80xc2x0 C., fr(min) is the minimum resonant frequency in the temperature range from xe2x88x9220 to 80xc2x0 C., and fr(20) is the frequency at 20xc2x0 C.
In an oscillator or the like using a quartz vibrator, an attempt has been made to cancel the temperature-change of the resonant frequency of the quartz vibrator with the temperature-changes of load capacitors.
This attempt is to provide an oscillation circuit with a temperature compensated circuit comprising capacitors and a thermistor, and to thereby cause this temperature compensated circuit to perform the function as a load capacitor that cancels the temperature-change of the resonant frequency of the quartz vibrator. This oscillation circuit is generally referred to as a temperature compensated crystal oscillator (TCXO).
Although it is possible to apply this method to an oscillator using an vibrator comprising a piezoelectric body having a bismuth layered compound as the main ingredient thereof, a problem occurs in that this results in an increase in the production cost and upsizing of the oscillator because of an increase in the number of components. If the temperature-change of the resonant frequency of a bismuth layered compound can be cancelled by utilizing only the temperature-change of the relative dielectric constant of a dielectric used as a load capacitor, it will become possible to easily improve the temperature change rate of resonant frequency.
It has been reported that some of bismuth layered compounds decrease in the resonant frequencies with the increase in temperature in the temperature range from xe2x88x9220 to 80xc2x0 C. (see, for example, Jpn. J. Appl. Phy., Vol. 38, Part 1, 9B, pp. 5557-5560). Since it is well known that the resonant frequency of an oscillator increases with the decrease in load capacitance, it can be expected that the temperature-change of the resonant frequency of a piezoelectric oscillator using such a bismuth layered compound can be cancelled by using, as load capacitors, dielectric bodies that decrease in the relative dielectric constant with the increase in temperature at the temperature range from xe2x88x9220 to 80xc2x0 C. However, no concrete examination has been made as far as the inventors of the present application know.
To solve the above-described problems, the present invention provides an oscillator having a feedback circuit and amplifiers using a vibrator that comprises a piezoelectric body having a bismuth layered compound as the main ingredient thereof. This oscillator comprises load capacitors comprising a feedback circuit together with the vibrator. The load capacitors each have as the main ingredient thereof, a dielectric body which decreases in a relative dielectric constant with an increase in temperature in a region not less than one half a temperature range from xe2x88x9220 to 80xc2x0 C., and in which the xcex5-TC is not less than 5000 ppm/xc2x0 C., wherein the xcex5-TC is an average change rate of the relative dielectric constant in the temperature range from xe2x88x9220 to 80xc2x0 C. and is represented by (Cmaxxe2x88x92Cmin)/(C20xc2x7100), where Cmax is the maximum value of capacitance in the temperature range from xe2x88x9220 to 80xc2x0 C., Cmin is the minimum value of capacitance in the temperature range from xe2x88x9220 to 80xc2x0 C., and C20 is the capacitance at 20xc2x0 C.
According to these arrangements, by setting the xcex5-TC of each of the load capacitors to be not less than 5000 ppm/xc2x0 C., it is possible to improve the temperature stability of oscillation frequency by the feedback circuit and amplifiers using the vibrator comprising the piezoelectric body having a bismuth layered compound as the main ingredient thereof.
The present invention also provides a method for producing an oscillator having a feedback circuit and amplifiers using a vibrator that comprises a piezoelectric body having a bismuth layered compound as the main ingredient thereof. This method comprises the step of using load capacitors which comprise a feedback circuit together with a vibrator. The load capacitors each have, as the main ingredient thereof, a dielectric body which decreases in the relative dielectric constant with the increase in temperature in the region not less than one half the temperature range from xe2x88x9220 to 80xc2x0 C., and in which the xcex5-TC is not less than 5000 ppm/xc2x0 C., wherein the xcex5-TC is an average change rate of the relative dielectric constant in the temperature range from xe2x88x9220 to 80xc2x0 C. and is represented by (Cmaxxe2x88x92Cmin)/(C20xc2x7100), where Cmax is the maximum value of capacitance in the temperature range from xe2x88x9220 to 80xc2x0 C., Cmin is the minimum value of capacitance in the temperature range from xe2x88x9220 to 80xc2x0 C., and C20 is the capacitance at 20xc2x0 C.
According to this method, by using the load capacitors each having, as the main ingredient thereof, a dielectric body of which the xcex5-TC is not less than 5000 ppm/xc2x0 C., it is possible to improve the temperature stability of oscillation frequency by the feedback circuit and amplifiers using the vibrator comprising the piezoelectric body having a bismuth layered compound as the main ingredient thereof.
In the above arrangements and method, the reason why the xcex5-TC, which is an average change rate of the relative dielectric constant in the temperature range from xe2x88x9220 to 80xc2x0 C., is set to be not less than 5000 ppm/xc2x0 C. is because, if it were not, a sufficient effect on the improvement in the temperature-change of the oscillation frequency would not be achieved.
Preferably, a bismuth compound used as a vibrator is at least one compound selected from the group consisting of Bi2WO6, CaBi2Nb2O9, SrBi2Nb2O9, BaBi2Nb2O9, PbBi2Nb2O9, CaBi2Ta2O9, SrBi2Ta2O9, BaBi2Ta2O9, PbBi2Ta2O9, Bi3TiNbO9, Bi3TiTaO9, Bi4Ti3O12, SrBi3Ti2NbO12, BaBi3Ti2NbO12, PbBi3Ti2NbO12, CaBi4Ti4O15, SrBi4Ti4O15, BaBi4Ti4O15, PbBi4Ti4015, Na05Bi45Ti4O15, K05Bi45Ti4O15, Ca2Bi4Ti5O18, Sr2Bi4Ti5O18, Ba2Bi4Ti5O18, Pb2Bi4Ti5O18, Bi6Ti3WO18, Bi7Ti4NbO21, and Bi10Ti3W3O30.
It is preferable that the above-described load capacitors each comprise a dielectric ceramic composition having, as the main ingredient thereof, a ceramic composition with a Perovskite type crystal structure.
Also, preferably, the load capacitors each comprise a dielectric ceramic composition having, as the main ingredient thereof, a ceramic composition having a composition formula of (Ba1-xSrx)TiO3 where x=0.4 to 0.7. The reason why it is preferable that the value of x in the above-described composition formula be in the range from 0.4 to 0.7 is because, outside this range, a sufficient effect of the present invention would not be achieved.
The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.